Of the xylene isomers, i.e. ortho-, meta-, and para-xylenes, the para-xylene isomer is of particular value as a large volume chemical intermediate. One method for manufacturing para-xylene is by disproportionation of toluene into xylenes. However, a disadvantage of this process is that large quantities of benzene are also produced. Another process for manufacturing para-xylene is the isomerization of a feedstream that contains non-equilibrium quantities of mixed ortho- and meta-xylene isomers and is lean with respect to para-xylene content. A disadvantage of this process is that the separation of the para-xylene from the other isomers is expensive.
There is growing interest in the alkylation of toluene with methanol as a next generation method for producing para-xylene. This technology can theoretically produce twice the yield of para-xylene from toluene, compared to the toluene disproportionation process. Examples of such toluene methylation processes include U.S. Pat. No. 3,965,207, which involves the methylation of toluene with methanol using a molecular sieve catalyst such as ZSM-5. U.S. Pat. No. 4,670,616 involves the production of xylenes by the methylation of toluene with methanol using a borosilicate zeolite catalyst which is bound by a binder such as alumina, silica, or alumina-silica.
A disadvantage of known toluene methylation catalysts is that methanol selectivity to para-xylene, the desirable product, has been low, in the range of 50-60%. The balance is wasted on the production of coke and other undesirable products. Attempts to increase the para-xylene selectivity have been conducted, however, it has been found that as para-xylene selectivity increases, the lifespan of the catalyst decreases. It is believed that the rapid catalyst deactivation is due to build up of coke and heavy by-products on the catalyst. The limited catalyst lifespan typically necessitates the use of a fluidized bed reactor wherein the catalyst is continuously regenerated. However, such a system usually requires high capital investment. A preferred system for toluene methylation is to use a fixed bed reactor because of lower capital investment. However, until a suitable catalyst is found that provides a sufficient lifespan, with a sufficient selectivity to the desired product, fixed bed systems are simply impractical. There remains a need for an improved toluene methylation process having a catalytic system that minimizes or avoids the disadvantages of prior systems, which includes low para-xylene selectivity and rapid catalyst deactivation. The present invention satisfies this need.